Conventional filter membranes (for ultrafiltration, microfiltration, . . . ) are tubular membranes having porous walls, with an inside or an outside skin or with two skins (where the term "skin" designates a thin surface coating which is less porous than the remainder of the wall). These membranes are obtained by extruding a polymer solution in the form of a hollow fiber, and by precipitating the polymer (e.g. polysulfone). The resulting continuous hollow fiber is wound into a skein which may optionally be placed in a perforated sleeve, and the ends of the skein are potted in plates of resin used for mounting in a housing, with the end portions of the skein then being cut off in order to transform the remainder into a bundle of fibers which are open at both ends. The assembly is then mounted in a housing having inlet and outlet orifices in order to constitute a filter module.
The manufacture of such modules is thus lengthly and requires a plurality of steps including, inter alia, handling the fiber as it is formed, thereby running the risk of breaking it.